In response to the recent demand for energy savings, an increasing number of low-pressure mercury discharge lamps, which have high-efficiency and long rated life, are replacing conventionally used incandescent lamps.
A compact self-ballasted fluorescent lamp (hereinafter, referred to simply as a “lamp”) is roughly composed of an arc tube, a lighting unit for operating the arc tube, and a resinous case for holding the arc tube and housing the lighting unit. At one end of the case is a base for attachment of the lamp to a socket of a lighting fixture. Power is supplied via the base from a commercial power source.
An exemplary lighting unit is of a so-called inverter type and is roughly composed of a rectifier/smoothing circuit portion having a diode bridge element and a smoothing capacitor, an inverter circuit portion having a pair of switching elements, and a resonant circuit portion having a choke coil and a resonant capacitor. Generally, the smoothing capacitor is constructed of an electrolytic capacitor, whereas the resonant capacitor is constructed of a film capacitor.
Each circuit portion of the lighting unit having the above structure is composed of a plurality of electronic components. In the event that any of electrodes of an arc tube or any electronic component (hereinafter, referred to simply as “heat generating component”) undergoes an excessive temperature rise for some reason, the resulting heat may cause various problems, such as deformation, discoloration, and meltdown of the case.
In order to prevent occurrence of such problems, it is suggested to terminate the circuit operation upon abnormal heat generation of an electronic component. For example, Patent Document 1 listed below discloses a lighting unit in which a smoothing capacitor is disposed in the vicinity of a choke coil that would generate heat when an electrode of the arc tube approaches the end of its useful life. Thus, when the electrode of the arc tube comes to the end of its useful life, the choke coil generates heat and the resulting heat breaks down the electrolytic capacitor. As a result, the circuit operation stops.
In another example, Patent Document 2 also listed below discloses a lighting unit having a thermal fuse disposed in a circuit. The thermal fuse burns out at a predetermined temperature, so that operation of the lighting unit is terminated.    Patent Document 1: JP Patent Application Publication No. 2004-303619    Patent Document 2: JP Patent Application Publication No. 11-3795